A gun such as a cannon or the like is usually adapted to fire either cased or caseless ammunition. A gun having a firing chamber adapted to fire caseless ammunition is provided with an obturation assembly for sealing the rear end of the firing chamber when a propellant charge is combusted inside the firing chamber to launch a projectile. In the instance of guns adapted to fire cased ammunition, such sealing is effected by the case of the ammunition itself held in position by a breechblock.
A conventional obturation assembly, such as the one described in U.S. Pat. No. 2,482,865, comprises a spindle having a shaft and a mushroom shaped head; and an obturation set including a resilient obturator pad for mating with and sealing against an obturation seat, an inner ring and two retaining rings. The obturation seat is usually defined by the rim of the mouth at the rear end of the firing chamber. When the spindle is displaced axially relative to the barrel of the gun by the combustion of the propellant charge in the firing chamber, the obturator pad deforms radially outwardly into sealing engagement with the obturation seat with the retaining rings limiting deformation of the obturator pad and preventing extrusion of the obturator pad.
The obturation assembly is usually held in position by the breechblock of the gun and the shaft of the spindle usually extends through a hole in a carrier abutting the breechblock. A chamber for a cased primer for igniting the propellant charge, is provided in the shaft and a so called primer head space is defined between the rear end of the primer case and the breechblock.
When installing the obturation assembly, it is known to provide a shim, from a set having a range of different thicknesses, between the rear end of the obturation set and the breechblock in order to adjust the position of the obturator pad relative to the obturation seat and to optimise the mating of the obturation set with the obturation seat.
A disadvantage of the conventional method of adjusting the obturation set is that because the spindle abuts the obturation set directly, adjustment of the position of the obturation set, adjusts the position of the obturation spindle accordingly and the primer head space is thus varied. When the primer head space becomes too large, the primer case tends to slide out of the primer chamber thus allowing propellant gas to escape from the firing chamber past the primer.
The specification of patent number CH 1579 A (Schneider) dated Nov. 16, 1889 reveals an obturation assembly comprising an obturation pad located between two shims, two split rings and four separate resilient retaining rings. Some of the disadvantages of an obturation assembly of this type are that it is not suitable for use with a gun of the type having a breechblock and a cased primer for igniting the propellant charge.